Multidrug resistance (MDR) efflux pumps in pathogenic microorganisms nullify the effects of antimicrobial drugs used in medicine. We have conducted phylogenetic analyses showing that these efflux pumps are associated with five superfamilies of transport systems. One of these, the drug/metabolite transporter (DMT) superfamily includes a family of small multidrug resistance (SMR)-conferring proteins that are discussed in detail in this review. A single microorganism such as Bacillus subtilis may possess multiple homologs of this family, and these homologs are believed to form both homo-oligomeric or hetero-oligomeric pumps, some of which export cationic drugs. The characteristics of some of these systems and the genes that encode them are described, with emphasis on the eight homologs encoded within the B subtilis genome. Anomalies and unanswered questions that provide impetus for future studies are presented.